1. Field of the Invention
The present disclosure relates to the technology of polysilicon crystallization, more specifically, relates to a device and a method for improving crystallization ratio of polysilicon.
2. Description of the Related Art
Polysilicon is popular for the thin film transistor manufacturing, particularly for the thin film transistors drive liquid crystal display due to the better electrical characteristics thereof than that of amorphous silicon and to the lower cost thereof than that of monocrystalline silicon.
In related art, the excimer laser annealing is popularly applied to the poly-silicon thin film.
U.S. Pat. No. 5,529,951 discloses a method of forming a polycrystalline silicon thin film improved in crystallinity and a channel of a transistor superior in electrical characteristics by the use of such a polycrystalline silicon thin film. An amorphous silicon layer of a thickness preferably of 30 nm to 50 nm is formed on a substrate. Next, substrate hearting is performed to set the amorphous silicon layer to preferably 350° C. to 500° C., more preferably 350° C. to 450° C. Then, at least the amorphous silicon layer is irradiated with laser light of an excimer laser energy density of 100 mJ/cm2 to 500 mJ/cm2, preferably 280 mJ/cm2 to 330 mJ/cm2, and a pulse width of 80 ns to 200 ns, preferably 140 ns to 200 ns, so as to directly anneal the amorphous silicon layer and form a polycrystalline silicon think film. The total energy of the laser used for the irradiation of excimer laser light is at least 5 J, preferably at least 10 J. However, the temperature generated by energy of the laser energy can not reach the melting temperature. It is necessary to perform multiple times of excimer laser irradiation to transform the amorphous silicon into polysilicon. When using a relatively big peak power laser to irradiate the surface, where peak power equals to the laser energy is divided by pulse width, the hydrogen explosion incurs easily, which makes polysilicon peeling off, impairs the crystallization ratio of polysilicon and pollutes the optical lens.
Chinese Patent No. CN 1822334A discloses a method of manufacture process of polysilicon thin film transistor. After the amorphous silicon layer is formed on the substrate, the amorphous silicon layer is patterned to formed silicon-based pattern to realize the preconfigured active area of the component. Next, a single-frequency long-pulse laser beam with pulse width from 100 nm to 200 nm is applied to prolong the heating time of the amorphous silicon. The melt crystallization of the amorphous silicon is prolonged to promote forming the crystalline grain. However, it does not solve the problem of the hydrogen explosion, in which the irradiation of a large peak power laser results. And it does not either solve the problem of the polysilicon peeling off impairing the crystallization ratio of polysilicon.
U.S. Pat. No. 7,009,140B2 has disclosed a high energy, high repetition rate workpiece surface heating method and apparatus comprise a pulsed XeF laser operating at or above 4000 Hz and producing a laser output light pulse beam at a center wavelength of about 351 nm; an optical system narrowing the laser output light pulse beam to less than 20 μm in a short axis of the laser output light pulse beam and expanding the laser output light pulse beam to form in a long axis of the beam a workpiece covering extent of the long axis; the optical system including a filed stop intermediate the laser and the workpiece; the workpiece comprising a layer to be heated; wherein the optical system focuses the laser output light pulse beam at a field stop with a magnification sufficient to maintain an intensity profile that has sufficiently steep sidewalls to allow the field stop to maintain a sufficiently steep beam profile at the workpiece without blocking the beam profile at too high an intensity level. The apparatus may also have a high average power in the laser output light pulse beam as delivered to the workpiece and a linebow correction mechanism in a short axis optical assembly. The linebow correction mechanism may comprise a plurality of weak cross cylinders. The system may comprise a catadioptric projection system. The linewidth due to laser diffraction and divergence may be less than geometric limitations. The system may project adjacent peaks of the nominal XeF spectrum to improve overall depth of focus through the separate center wavelengths of each respective adjacent peak having a different focal plane at the workpiece. The system may comprise a linebow is correction mechanism within a field stop optical assembly correcting linebow at the field stop plane and within a workpiece projection optical assembly correcting linebow at the work piece plane. However, the laser pulse width is relatively long in this patent. The laser energy is large, and the peak power of the laser in the above invention is relatively large. Consequently, the above invention did not solve the problems of the hydrogen explosion and the hydrogen explosion makes polysilicon peeling off to affects crystallization ratio of polysilicon.